Counters are, in essence, variables maintained by CSS whose values may be incremented by CSS rules to track how many times they're used.

• counter-reset is used to initialize a counter, the name of which is the value of the attribute. By default, the counter starts at 0. This property can also be used to change its value to any specific number.
• counter-increment is used for an element that will be countable. Once counter-reset is initialized, a counter's value can be increased or decreased.
• counter(name, style) displays the value of a section counter. Generally used with the content property. This function can receive two parameters, the first being the name of the counter and the second one either decimal or upper-roman (decimal by default).
• counters(counter, string, style) displays the value of a section counter. Generally used with the content property. This function can receive three parameters, the first as the name of the counter, the second one you can include a string which comes after the counter and the third one can be decimal or upper-roman (decimal by default).
• A CSS counter can be especially useful for making outlined lists, because a new instance of the counter is automatically created in child elements. Using the counters() function, separating text can be inserted between different levels of nested counters.
• Note that you can create an ordered list using any type of HTML.

<ul>
                                                            <li>List item</li>
                                                            <li>List item</li>
                                                            <li>
                                                              List item
                                                              <ul>
                                                                <li>List item</li>
                                                                <li>List item</li>
                                                                <li>List item</li>
                                                              </ul>
                                                            </li>
                                                          </ul>

ul {
                                                            counter-reset: counter;
                                                          }
                                                          
                                                          li:before {
                                                            counter-increment: counter;
                                                            content: counters(counter, '.') ' ';
                                                          }